Packing respiring biological materials with atmosphere control member

ABSTRACT

Packaging of fruit and vegetables, and other respiring biological materials, makes use of an atmosphere-control member comprising a gas-permeable membrane and an apertured cover member over the membrane. The combination results in a control member having a ratio of CO 2  transmission rate to O 2  transmission rate which is lower than the same ratio for the gas-permeable membrane. This is particularly useful for materials which are preferably stored in an atmosphere containing a relatively high proportion of CO 2 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the packaging of biological materials, especially fresh produce.

2. Introduction to the Invention

Fruit and vegetables, and other respiring biological materials, consume oxygen (O₂) and produce carbon dioxide (CO₂) at rates which depend upon temperature and upon the particular material and the stage of its development. Their storage stability depends on the relative and absolute concentrations of O₂ and CO₂ in the atmosphere surrounding them, and on temperature. Ideally, a respiring material should be stored in a container having a total permeability to O₂ and a total permeability to CO₂ which are correlated with (i) the atmosphere outside the package (usually air), (ii) the rates at which the material consumes O₂ and produces CO₂, and (iii) the temperature, to produce an atmosphere within the container (the "packaging atmosphere") having the desired O₂ and CO₂ concentrations for preservation of the material. The total permeability to water vapor may also be significant. This is the principle behind the technology of controlled atmosphere packaging (CAP) and modified atmosphere packaging (MAP), as discussed, for example, in U.S. Pat. Nos. 4,734,324 (Hill), 4,830,863 (Jones), 4,842,875 (Anderson), 4,879,078 (Antoon), 4,910,032 (Antoon), 4,923,703 (Antoon), 5,045,331 (Antoon), 5,160,768 (Antoon) and 5,254,354 (Stewart), copending, commonly assigned U.S. patent application Ser. No. 08/759,602 filed Dec. 5, 1996 (Docket No. 10621.2 US), published as International Publication No. WO 96/38495 (Application No. PCT/US96/07939), and European Patent Applications Nos. 0,351,115 and 0,351,116 (Courtaulds). The disclosure of each of these documents is incorporated herein by reference.

The O₂ transmission rate (referred to herein as OTR) and CO₂ transmission rate (referred to herein as COTR), of a body composed of a particular material, are the amounts of O₂ and CO₂, respectively, which will pass through a defined area of that body under defined conditions. The total permeabilities of a container to O₂ and CO₂ depend, therefore, upon the areas, OTRs and COTRs of the various parts of the container.

The preferred packaging atmosphere depends on the stored material. For many materials, the preferred concentration of O₂ is less than the preferred concentration of CO₂. For example, broccoli is generally best stored in an atmosphere containing 1-2% O₂ and 5-10% CO₂ ; berries are generally best stored in an atmosphere containing 5-10% O₂ and 10-20% CO₂ ; and cherries are generally best stored in an atmosphere containing 5-8% O₂ and 10-20% CO₂. In order to produce a packaging atmosphere having a high ratio of CO₂ to O₂, the container should have a low ratio of total CO₂ permeability to total O₂ permeability. The term R ratio is used herein to denote the ratio of COTR to OTR for a particular material or the ratio of total CO₂ permeability to total O₂ permeability of a container or part of a container.

Respiring biological materials are normally stored at temperatures substantially below normal room temperature, but are often exposed to higher temperatures before being used. At such higher temperatures, the respiration rate increases, and in order to maintain the desired packaging atmosphere, the permeability of the container preferably increases sharply between storage temperatures and room temperature.

Respiring biological materials are generally stored in sealed polymeric containers. Conventional polymeric films, when used on their own, do not provide satisfactory packaging atmospheres because their OTR and COTR values are very low and their R ratios are high. Microporous polymeric films, when used on their own, are also unsatisfactory, but for different reasons; namely because their OTR and COTR values are very high and their R ratios close to 1.0. It has been proposed, therefore, to make use of containers which comprise

(i) one or more barrier sections which are relatively large in area and are composed of materials having relatively low OTR and COTR values (e.g. are composed of a conventional polymeric film), and

(ii) one or more atmosphere-control members which are relatively small in area and are composed of a microporous film, and which provide at least a large proportion of the desired permeability for the whole container.

However, for containers of conventional size, the preferred total O₂ permeability, although larger than can be provided by the barrier sections alone, is still so small that the control members need to be very small in area. Such very small control members are difficult to incorporate into containers, and can easily become blocked in use. In addition, the OTR of microporous films does not change much with temperature.

As described in copending commonly assigned application Ser. No. 08/759,602 and corresponding International Publication No. WO 96/38495 (referenced above), much improved results can be obtained through the use of atmosphere-control members composed of a membrane prepared by coating a thin layer of a polymer onto a microporous film. The OTR of these membranes is such that the atmosphere-control members are of practical size. Furthermore, through appropriate choice of the coating polymer, the membranes can have OTRs which increase sharply with temperature. However, although the membranes are very satisfactory for many purposes, they often have R ratios which are higher than is optimal when the desired packaging atmosphere contains a relatively large proportion of CO₂.

SUMMARY OF THE INVENTION

I have discovered that if a gas-permeable membrane is covered, on the side exposed to the air, by a relatively gas-impermeable cover member having one or more small apertures therein, the dimensions of the aperture(s) have important and surprising effects on the permeability characteristics of the combination of the membrane and the cover member. In particular, I have discovered that the R ratio of the combination can be substantially less than the R ratio of the membrane itself. The invention is, therefore, particularly useful for containers used for storing materials which are preferably stored in an atmosphere containing a relatively high proportion of CO₂.

In a first preferred aspect, this invention provides a container which

(a) comprises

(i) one or more barrier sections which are relatively impermeable to O₂ and CO₂, and

(ii) one or more atmosphere-control members which are relatively permeable to O₂ and CO₂ ; and

(b) can be sealed around a respiring biological material to provide a sealed package which is surrounded by air and which contains a packaging atmosphere around the biological material;

at least one said control member comprising

(a) a gas-permeable membrane; and

(b) an apertured cover member which, when the container has been sealed around a respiring biological material to provide a said sealed package, lies between the gas-permeable membrane and the air surrounding the package;

the gas-permeable membrane having, in the absence of the apertured cover member,

(i) an O₂ permeability, referred to herein as OTR_(perm), of at least 155,000 ml/m² •atm•24 hr. (10,000 cc/100 in² •atm•24 hr.), and

(ii) a permeability ratio of COTR to OTR, referred to herein as R_(perm) ; and

the apertured cover member being composed of

(i) a barrier portion having an O₂ permeability, referred to herein as OTR_(bar), which is less than 0.5 times OTR_(perm), and

(ii) an aperture portion through which the gas-permeable membrane is exposed to the air surrounding the package, the aperture portion being such that the control member has a permeability ratio of COTR to OTR, referred to herein as R_(control), of at most 0.9 times R_(perm).

In a second preferred aspect, this invention provides a package which is stored in air and which comprises

(a) a sealed container, and

(b) within the sealed container, a respiring biological material and a packaging atmosphere around the biological material;

said container being a container as defined above which has been sealed around the biological material.

In a third preferred aspect, this invention provides an atmosphere-control member suitable for incorporation into a container according to the first aspect of the invention, said atmosphere-control member comprising

(a) a gas-permeable membrane; and

(b) an apertured cover member;

the gas-permeable membrane having, in the absence of the apertured cover member

(i) an O₂ permeability, OTR_(perm), of at least 155,000 ml/m² •atm •24 hr. (10,000 cc/100 in² •atm•24 hr.), and

(ii) a permeability ratio, R_(perm) ; and

the apertured cover member being composed of

(i) a barrier portion having an O₂ permeability, OTR_(bar), which is less than 0.5 times OTR_(perm), and

(ii) an aperture portion through which the gas-permeable membrane is exposed to the air surrounding the package, the aperture portion being such that the control member has a permeability ratio, R_(control), of at most 0.9 times R_(perm).

BRIEF DESCRIPTION OF THE DRAWING

The invention is illustrated in the accompanying drawings, in which

FIGS. 1 and 2 are diagrammatic illustrations of a part of a package of the invention, and

FIGS. 3 and 4 are diagrammatic illustrations of the test set-up used in the Examples.

DETAILED DESCRIPTION OF THE INVENTION

In describing the invention, the following abbreviations, definitions, and methods of measurement are used. OTR is O₂ permeability; OTR_(perm) is the OTR of the gas-permeable membrane in the absence of the cover member; OTR_(bar) is the OTR of the barrier portion of the cover member; and OTR_(control) is the OTR of the atmosphere-control member. COTR is CO₂ permeability; COTR_(perm) is the COTR of the gas-permeable membrane in the absence of the cover member; COTR_(bar) is the COTR of the barrier portion of the cover member; and COTR_(control) is the COTR of the atmosphere-control member. OTR and COTR values are measured at about 22° C. unless otherwise noted, and given in ml/m² •atm•24 hr, with the equivalent in ccl/100 inch² •atm•24 hr. given in parentheses. OTR and COTR values given herein were measured as described below in connection with FIGS. 3 and 4. The abbreviation P₁₀ is used to denote the ratio of OTR at a first temperature T₁ ° C. (OTR₁) to OTR at a second temperature T₂ ° C. (OTR₂), where T₂ is (T₁ -10)° C., T₁ being a temperature in the range 10-25° C.; or, when T₂ is a temperature which is not (T₁ -10)° C., but is a temperature lower than T₁, to denote the ratio ##EQU1## The abbreviation R is used to denote the ratio of COTR to OTR; thus R_(perm) is COTR_(perm) /OTR_(perm), and R_(control) is COTR_(control) /OTR_(control). Pore sizes given in this specification are measured by mercury porosimetry or an equivalent procedure. Percentages are by volume except where otherwise noted. For crystalline polymers, the abbreviation T_(o) is used to denote the onset of melting, the abbreviation T_(p) is used to denote the crystalline melting point, and the abbreviation ΔH is used to denote the heat of fusion. T_(o), T_(p) and ΔH are measured by means of a differential scanning calorimeter (DSC) at a rate of 10° C./minute and on the second heating cycle.

The novel atmosphere-control members of the invention, in use, form part of a container which is sealed around a respiring biological material. The container can contain a single novel control member, or two or more novel control members (which will usually be the same, but can be different). In some cases, the container can contain a pinhole in order to ensure equalization of the external air pressure and the internal pressure within the container. The invention includes the possibility that the container also contains one or more atmosphere-control members whch are not in accordance with the present invention.

The remainder of the container; i.e. the barrier section or sections, which is of much larger area than the control member(s), is composed of one or more materials which are relatively impermeable to O₂ and CO₂ ; e.g. a suitable polymeric film or other shaped article. In some cases, the barrier sections are composed of a material whose OTR and COTR are so low that the packaging atmosphere is substantially determined only by the control member(s). In other cases, the barrier sections have OTR and COTR values which (although low) are high enough that, having regard to the relative large area of the barrier section(s), a substantial proportion of the O₂ entering the packaging atmosphere passes through the barrier sections. At 22° C., this proportion can be, for example, as high as 50%, but is generally less than 25%. Typically, the barrier section is provided by a bag of flexible polymeric film or by two preformed, relatively rigid, polymeric members which have been heat-sealed to each other, and the atmosphere-control member covers an aperture cut into the bag or one of the preformed members. The control member(s) can be secured to the barrier section(s) in any way, for example through heat sealing or with an adhesive.

The size and nature of the gas-permeable membrane, and the number and dimensions of the aperture(s) in the cover member, together determine the absolute and relative amounts of O₂ and CO₂ which can enter and leave the container, and, therefore, the packaging atmosphere within the container. The desired packaging atmosphere will depend upon the biological material within the container, and the temperature, and the atmosphere-control member(s) should be selected accordingly. Those skilled in the art of packaging biological materials will have no difficulty, having regard to the disclosure in this specification and their own knowledge, in designing containers which will give substanially improved results under practical conditions of use and which can be economically manufactured.

The overall size and shape of the control member should be such that the control member can be easily handled and secured to the rest of the container, and positioned on the container so that it will not be damaged or blocked during the packaging operation or during storage. Typically, the control member will be rectangular in shape, with each side of the rectangle being 1 to 4 in (25 to 100 mm). However, other shapes and sizes can be used. The overall dimensions of the gas-permeable membrane and of the cover member will normally be the same as the overall dimensions of the control member. The cover member can be an integral part of a larger member which also provides the barrier sections which surround the control member, for example a polymeric film having a central area to which the gas-permeable membrane is secured and a peripheral area which is part of the barrier section, as illustrated, for example, in FIGS. 1 and 2.

The gas-permeable membrane must have an OTR sufficiently high that having regard to the area of the membrane itself and the number and dimensions of the apertures in the cover member, sufficient O₂ is admitted into the container. OTR_(perm) is, therefore, at least 155,000 ml/m² •atm•24 hr (10,000 cc/100 in² •atm•24 hr), preferably at least 310,000 ml/m² •atm•24 hr (20,000 cc/100 in² •atm•24 hr), particularly at least 775,000 ml/m² •atm•24 hr (50,000 cc/100 in² •atm•24 hr). On the other hand, OTR_(perm) should not be too high, since the size of the control member then becomes smaller than is desirable. OTR_(perm) is, therefore preferably less than 3,100,000 ml/m² •atm•24 hr (200,000 cc/100 in² •atm•24 hr), preferably 387,000 to 2,325,000 ml/m² •atm•24 hr (25,000 to 150,000 cc/100 in² •atm•24 hr), particularly 774,000 to 2,325,000 ml/m² •atm•24 hr (50,000 to 150,000 cc/100 in² •atm•24 hr).

As noted above, the R ratio of the atmosphere-control member (R_(control)) is substantially less than the R ratio of the gas-permeable membrane alone (R_(perm)). But of course, the value of R_(perm) is an important factor in determining R_(control). R_(perm) is usually at least 2, preferably at least 4, e.g. 4 to 6, and can be much higher, for example up to 12. Membranes having high P₁₀ values (i.e. whose permeability increases sharply with temperature) often have high R ratios, and one of the advantages of the present invention is that by using such membranes in combination with an apertured cover member, it is possible to produce atmosphere-control members having novel and valuable combinations of relatively low R ratios and relatively high P₁₀ values. For the production of control members having high P₁₀ values, the gas-permeable membrane, in the absence of the cover member, should have a comparably high P₁₀ value; e.g., at least 1.3, preferably, or at least 2.6, over at least one 10° C. range between -5 and 25° C.

Gas-permeable membranes suitable for use in this invention include those described in detail in application Ser. No. 08/759,602 and corresponding International Publication No. PCT/US96/07939 (referenced above), in particular those having an R ratio of at least 2, preferably at least 4. Thus, preferred gas-permeable membranes for use in this invention comprise

(a) a microporous polymeric film, and

(b) a polymeric coating on the microporous film, the polymeric coating changing the permeability of the microporous film so that the membrane

(i) has a P₁₀ ratio, over at least one 10° C. range between -5 and 15° C., of at least 1.3, preferably at least 2.6;

(ii) has an oxygen permeability (OTR), at all temperatures between 20° and 25° C., of at least 775,000 ml/m² •atm•24 hr (50,000 cc/100 inch² •atm•24 hr), preferably at least 1,550,000 ml/m₂ •atm•24 hr (100,000 cc/100 in² •atm•24 hr); and

(iii) has a CO₂ /O₂ permeability ratio (R) of at least 2.0, preferably at least 4.0.

Preferably, the microporous film has at least one of the following characteristics

(1) it has an average pore size of less than 0.24 micron, at least 90% of the pores preferably having a size less than 0.24 micron;

(2) it has a tear strength of at least 30 g;

(3) it has a Sheffield Smoothness of at least 30;

(4) it comprises a polymeric matrix comprising an essentially linear ultrahigh molecular weight polyethylene having an intrinsic viscosity of at least 18 deciliters/g, or comprising an essentially linear ultrahigh molecular weight polypropylene having an intrinsic viscosity of at least 6 deciliters/g; and

(5) it comprises a finely divided, particulate, substantially insoluble filler which is distributed throughout the film.

Preferably, the coating polymer is coated at a coating weight of 1.7 to 2.9 g/m² and has one or more of the following characteristics

(1) it is a crystalline polymer having a T_(p) of -5 to 40° C., preferably 0 to 15° C., and a ΔH of at least 5 J/g, preferably at least 20 J/g;

(2) it is a side chain crystalline polymer, preferably one in which T_(p) -T_(o) is less than 10° C., for example a side chain crystalline polymer prepared by copolymerizing (i) at least one n-alkyl acrylate or methacrylate in which the n-alkyl group contains at least 12 carbon atoms and (ii) one or more comonomers selected from acrylic acid, methacrylic acid, and esters of acrylic or methacrylic acid in which the esterifying group contains less than 10 carbon atoms;

(3) it is cis-polybutadiene, poly(4-methylpentene), polydimethyl siloxane, or ethylene-propylene rubber; and

(4) it has been crosslinked.

For further details of suitable gas-permeable membranes, reference should be made to the document itself, which is incorporated herein by reference.

The barrier portion of the apertured cover member has an OTR (OTR_(bar)) which is substantially less than the OTR of the gas-permeable membrane (OTR_(perm)), e.g. less than 0.5 times OTR_(perm), preferably less than 0.05 times OTR_(perm), particularly less than 0.01 times OTR_(perm), and can be such that the barrier portion is substantially impermeable to O₂ and CO₂. The dimensions of the aperture(s) in the apertured cover member have a surprising effect on the permeability characteristics of the atmosphere-control member. As would be expected, (since only a small proportion of the gas-permeable membrane is directly exposed to the air) the absolute amounts of O₂ and CO₂ passing through the membrane are reduced. However, the reduction is not as great as would be expected and, more important, the R ratio of the combination of the membrane and the cover member (R_(control)) is unexpectedly lower than the R ratio of the membrane itself (R_(perm)). The extent of the reduction in the R ratio depends upon the proportion of the membrane which is exposed (i.e. the total area of the aperture or apertures) and the dimensions of the individual aperture(s). The desired reduction in R ratio depends upon the value of R_(perm) and the material to be packaged. R_(control) is at most 0.9 times R_(perm), preferably at most 0.8 times R_(perm). In many cases a substantially greater reduction. e.g. such that R_(control) is at most 0.5 times R_(perm), is desirable and can be achieved without difficulty. The value of (R_(perm) -R_(control)) is preferably at least 1.0, particularly at least 2.0.

The gas-permeable membrane has an area A_(perm), and the aperture portion of the cover member has an area, A_(open). A_(open) is generally at most 0.15 times A_(perm), preferably at most 0.04 times A_(perm). Depending on the desired reduction in R ratio, A_(open) is often less than 0.02 times A_(perm). The area of each aperture, A_(aperture) is also important. A_(aperture) is generally less than 0.35 in² (2.25 mm²), for example 0.015 to 0.15 in² (9.7 to 97 mm²), preferably 0.05 to 0.15 in² (32 to 97 mm²), again depending on the desired reduction in R ratio. The apertures can be of any convenient shape, e.g. circular, oval, or irregular. The periphery of each aperture generally has a length less than 2 in (51 mm), for example 0.14 to 1.4 in (3.5 to 35 mm), preferably 0.8 to 1.4 in (20 to 35 mm).

The apertures can be produced in the cover member by completely removing a portion of the initial film. However, I have obtained more consistent results when all but a small part of the periphery of the aperture is cut through, and the resulting flap is folded so that it lies between the gas-permeable membrane and the cover member. This results in a "tented" configuration which increases the area of the membrane which is directly exposed to the air. I have observed that in some cases, even when there is no flap of this kind, after a period of equilibration, the gases entering and leaving the container produce a small separation between the membrane and the cover member around the periphery of the aperture, resulting in a similar configuration.

Referring now to the drawings, FIG. 1 is a plan view of part of a package of the invention, and FIG. 2 is a cross section taken on line II, II of FIG. 1. Both FIG. 1 and FIG. 2 are diagrammatic in nature and are not to scale; in particular the thicknesses of the various films have been exaggerated in FIG. 2 in the interests of clarity. In FIGS. 1 and 2, the package comprises a sealed container 1 which contains strawberries 2. The container 1 is composed of barrier sections 11 of a substantially impermeable polymeric film and an atmosphere control member 12. The control member comprises a gas-permeable membrane 121 and an apertured cover member 122. The membrane 121 is heat sealed to the underside of the cover member 122, so that the cover member lies between the membrane and the air surrounding the sealed container. The apertured cover member is integral with the top barrier section 11 (and is, therefore, composed of a substantially impermeable polymeric film) and has an aperture 123 in the center thereof. The aperture 123 has been produced by cutting almost all the way around a circle, and folding the resulting flap under the cover member so that it lies between the gas-permeable membrane and the cover member.

FIG. 3 is a plan view of a part of the test set-up used in the Examples below, and FIG. 4 is a cross section taken on line IV--IV of FIG. 3. Like FIGS. 1 and 2, FIG. 3 and FIG. 4 are diagrammatic in nature and are not to scale. FIGS. 3 and 4 show an impermeable box 2 which is surrounded by air, which has an open top, and which is fitted with valves 21 and 22, an O₂ sensor 25, a CO₂ sensor 26, and a pressure sensor 27. An atmosphere-control member 12 which is to be tested is sealed over the open top of the box, using double-sided adhesive tape. The control member 12 comprises a gas-permeable membrane 121 and an apertured cover member 122 having one or more circular apertures 123 therein (a single aperture being shown in FIGS. 3 and 4). To test the control member, the box is first filled with a mixture of 15% CO₂, 3% O₂ and 82% nitrogen which is supplied through valve 21 and removed through valve 22 for a time sufficient to ensure that the desired gas mixture is present within the box. Valves 21 and 22 are then closed. The data generated by the O₂ sensor 25, the CO₂ sensor 26 and the pressure sensor 27 (as the gas mixture equilibrates with the air outside the box) are passed to a computer (not shown), and are used to calculate OTR and COTR values for the control member (based on the total area of the gas-permeable membrane), using the technique described in "Exponential Decay Method for Determining Gas Transmission Rate of Films" by L. Moyls, R. Hocking, T. Beveridge, G. Timbers (1992) Trans. of the ASAE 35:1259-1266.

The invention is illustrated by the following examples, which are summarized in the tables below. In each of the examples, the OTR and COTR of a gas-permeable membrane were measured as described in connection with FIGS. 3 and 4, and the R ratio was calculated. The membrane was then covered by an apertured cover member which had one or more round apertures in it. The OTR and COTR of the resulting covered membrane were measured, and the R ratio calculated. The tables below show the reduction in OTR (i.e. the difference between the OTR of the membrane on its own, OTR_(perm), and the OTR of the covered membrane, OTR_(control)) expressed as a percentage of OTR_(perm), and the reduction in R ratio (i.e. the difference between the R ratio of the membrane on its own, R_(perm), and the R ratio of the covered membrane, R_(control)) expressed as a percentage of R_(perm).

In each of the Examples, the gas-permeable membrane was a microporous film which had been coated with a polymer, and the cover member was prepared from a film which was substantially impermeable to O₂ and CO₂, as further identified below. It should be noted that the OTR and R ratio of the gas-permeable membrane vary by up to about 5% and that such variations in the results reported below should not be regarded as significant. The size of the membrane and the cover member was 51×76 mm (2×3 inch), except where noted.

EXAMPLE 1

In this example, the gas-permeable membrane was prepared by coating a copolymer of acrylic acid and a mixture of n-alkyl acrylates onto a microporous polyethylene film containing about 60% silica which is available from PPG Industries under the trade name Teslin SP7. The copolymer had a T_(p) of less than 5° C. At about 22° C., the membrane had an OTR of about 1,550,000 ml/m² •atm•24 hr (100,000 cc/100 in² •atm•24 hr) and an R ratio of about 5.7. At about 7° C., the membrane had an OTR of about 852,500 ml/m² •atm•24 hr (54,000 cc/100 in² •atm•24 hr) and an R ratio of about 6. The cover member was prepared from a coextruded polyethylene/polystyrene film sold under the trade name BF-915 by Barrier Films Corp., which had an OTR of about 5425 ml/m² •atm•24 hr (350 cc/100 in² •atm•24 hr). Table 1 shows the results at room temperature (22-25° C.) using the test set-up illustrated in FIGS. 3 and 4.

                  TABLE 1                                                          ______________________________________                                         Run  Holes In Cover  % of Membrane                                                                              % Drop                                                                               % Drop                                  No.  Diameter mm (inch)                                                                          No.    Covered   In OTR                                                                               In R                                  ______________________________________                                         1    12.7   (0.5) 4      86.91     -4.98 7.14                                  2     9.5   (0.375)                                                                              6      88.96     1.09  8.32                                  3     9.5   (0.375)                                                                              4      92.64     1.10  14.02                                 4     9.5   (0.375)                                                                              1      98.16     12.79 26.72                                 ______________________________________                                    

EXAMPLE 2

In this example, the gas-permeable membrane was as in Example 1, but the cover member was prepared from a polyethylene terephthalate film sold under the trade name Mylar OLAF 100 by du Pont, which has an OTR about 0.014 times the OTR of the BF-915 used in Example 1. Table 2 shows the results obtained at room temperature (about 22.5° C.) and 7° C., using the test set-up illustrated in FIGS. 3 and 4, and the P₁₀ values calculated from those results. The P₁₀ value of the membrane on its own is 1.42.

                                      TABLE 2                                      __________________________________________________________________________     Run                                                                               Holes In Cover                                                                             % in Membrane                                                                          % Drop In OTR                                                                            % Drop In R                                   No.                                                                               Diameter mm (inch)                                                                      No.                                                                               Covered at 22.5° C.                                                                   at 7° C.                                                                    at 22.5° C.                                                                   at 7° C.                                                                    P.sub.10                            __________________________________________________________________________     1  15.9                                                                               (0.625)                                                                             1  94.89   -2.16 -0.77                                                                              15.1  12.3                                                                               1.28                                2  9.5 (0.375)                                                                             1  98.16   9.35  8.24                                                                               23.5  8.95                                                                               1.40                                3  6.35                                                                               (0.25)                                                                              1  99.18   22.27 14.88                                                                              36.83 22.97                                                                              1.31                                4  3.18                                                                               (0.125)                                                                             1  99.80   34.45 33.8                                                                               63.23 43.99                                                                              1.30                                5  1.59                                                                               (0.0625)                                                                            1  99.95   42.89 63.73                                                                              75.72 52.69                                                                              1.36                                __________________________________________________________________________

EXAMPLE 3

In this example, the gas-permeable membrane was prepared by coating Teslin SP7 with polyethylene glycol methacrylate sold under the trade name MPEG 350 by International Specialty Chemicals. The membrane had an OTR of about 821,500 ml/m² •atm•24 hr (53,000 cc/100 in² •atm•24 hr) and an R ratio of about 11.84. The cover member was Mylar OLAF 100. Table 3 shows the results obtained at room temperature (about 22° C.) using the test set-up shown in FIGS. 3 and 4.

                  TABLE 3                                                          ______________________________________                                         Run  Holes In Cover  % of Membrane                                                                              % Drop                                                                               % Drop                                  No.  Diameter mm (inch)                                                                          No.    Covered   In OTR                                                                               In R                                  ______________________________________                                         1    9.5     (0.375)  1    98.16     19.56 22.79                               2    6.35    (0.25)   1    99.18     11.34 45.18                               3    3.18    (0.125)  1    99.80     41.66 75.55                               4    1.59    (0.0625) 1    99.95     38.27 77.69                               ______________________________________                                    

EXAMPLE 4

In this example, the gas-permeable membrane was the same as in Example 1. Samples of the membrane 5.1×5.1 mm (2×2 inch) were heat-sealed at the edges to cover members composed of Mylar OLAF 100 and having apertures. The resulting assemblies were used as atmosphere-control members on packages containing trays of 1.14 kg (2.5 lb) of whole strawberries, and the steady state atmosphere within the packages was monitored at intervals of 3 days over a period of 13 days at 5° C. TabLe 4 shows the average concentrations of O₂ and CO₂ and their range of variation, and the percentage drops in OTR and R.

                                      TABLE 4                                      __________________________________________________________________________     Run                                                                               Holes in Cover                                                                             % of Membrane                                                                                      % Drop                                                                             % Drop                                  No.                                                                               Diameter mm (inch)                                                                      No.                                                                               Covered % O.sub.2                                                                            % CO.sub.2                                                                           In OTR                                                                             In R                                    __________________________________________________________________________     1  9.5 (0.375)                                                                             4  88.96   10.5 ± 2.13                                                                       4.02 ± 1.1                                                                         0  10                                      2  6.35                                                                               (0.25)                                                                              1  98.77    6.9 ± 0.23                                                                       8.4 ± 0.18                                                                        27  35                                      3  9.5 (0.375)                                                                             4  88.96   11.4 ± 2.3                                                                        4.0 ± 0.83                                                                         0  10                                      4  6.35                                                                               (0.25)                                                                              1  98.77    8.8 ± 0.94                                                                       7.8 ± 0.64                                                                        23  33                                      __________________________________________________________________________ 

What is claimed is:
 1. A container which(a) is composed of(i) one or more barrier sections which are relatively impermeable to O₂ and CO₂, and (ii) one or more atmosphere-control members which are relatively permeable to O₂ and CO₂ ; and (b) can be sealed around a respiring biological material to provide a sealed package which is surrounded by air and which contains a packaging atmosphere around the biological material;at least one said control member comprising (a) a gas-permeable membrane composed of (i) a microporous film having an R ratio of about 1, and (ii) a polymeric coating on the microporous film; and (b) an apertured cover member which, when the container has been sealed around a respiring biological material to provide a said sealed package, lies between the gas-permeable membrane and the air surrounding the package; the gas permeable membrane having, in the absence of the apertured cover member,(i) an O₂ permeability, OTR_(perm), of at least 155,000 ml/m² •atm•24 hr (10,000 cc/100 in² •atm•24 hr), and (ii) a permeability ratio, R_(perm), of at least 2, and the apertured cover member being composed of(i) a barrier portion having an O₂ permeability, OTR_(bar), which is less than 0.5 times OTR_(perm), and (ii) an aperture portion which comprises at least one aperture having an area of at least 0.015 in² and through which the gas-permeable membrane is exposed to the air surrounding the package, the aperture portion being such that the control member has a permeability ratio, R_(control), of at most 0.9 times R_(perm).
 2. A container according to claim 1 wherein(a) R_(control) is(i) greater than 1.00 and (ii) at most 0.8 times R_(perm), (b) OTR_(bar) is less than 0.01 times OTR_(perm), and (c) the gas-permeable membrane has an area A_(perm), and the aperture portion of the cover member has an area A_(open) which is at most 0.04 times A_(perm).
 3. A container according to claim 1 wherein the aperture portion of the cover member consists of one or more apertures, each said aperture having an area, A_(aperture), less than 0.155 in².
 4. A container according to claim 1 wherein the aperture portion of the cover member consists of one or more apertures, each said aperture having a periphery whose length is less than 2 in (51 mm).
 5. A container according to claim 1 wherein OTR_(perm) is less than 3,100,000 ml/m² •atm•24 hr (200,000 cc/100 in² •atm•24 hr).
 6. A container according to claim 1 wherein(a) the microporous polymeric film has an O₂ permeability of at least 11,625,000 ml/m² •atm•24 hr (750,000 cc/100 in² •atm•24 hr), and (b) the polymeric coating on the microporous film is such that the gas-permeable membrane has an O₂ permeability, OTR_(perm), of 387,000 to 2,325,000 ml/m² •atm•24 hr (25,000 to 150,000 cc/100 in² •atm•24 hr).
 7. A container according to claim 6 wherein the polymeric coating is such that the gas-permeable membrane has a P₁₀ ratio, over at least one 10° C. range between -5 and 25° C., of at least 1.3.
 8. A container according to claim 7 wherein the gas-permeable membrane has a P₁₀ ratio of at least 2.6.
 9. A container according to claim 1 which contains a pinhole.
 10. A container according to claim 1 wherein at least 75% of the O₂ which enters the packaging atmosphere, after the container has been sealed around the biological material and while the sealed package is at 22° C., passes through said at least one atmosphere control member.
 11. A package which is stored in air and which comprises(a) a sealed container, and (b) within the sealed container, a respiring biological material and a packaging atmosphere around the biological material;said container being a container as defined in claim 1 which has been sealed around the biological material.
 12. A package according to claim 11 wherein the biological material is selected from cherries, strawberries, raspberries, blueberries, nectarines and peaches.
 13. A method of packaging a respiring biological material which comprises(A) placing the biological material in a container as defined in claim 1, and (B) sealing the container around the biological material.
 14. A container according to claim 1 wherein the polymer coating on the microporous film has a coating weight of 1.7 to 2.9 g/m² and is composed of a crystalline polymer having a crystalline melting point, T_(p), of -5° to 40° C. and a heat of fusion, αH, of at least 5 J/g.
 15. A container according to claim 14 wherein T_(p) is 0° to 15° C. and ΔH is at least 20 J/g.
 16. A container according to claim 14 wherein the crystalline polymer is a side chain crystalline polymer having an onset of melting temperature, T_(o), such that T_(p) -T_(o) is less than 10° C.
 17. A container according to claim 16 wherein the crystalline polymer has been crosslinked.
 18. A container according to claim 1 wherein the polymer coating on the microporous film has a coating weight of 1.7 to 2.9 g/m² and is composed of a polymer selected from the group consisting of cis-polybutadiene, poly(4-methylpentene), polydimethyl siloxane, and ethylene-propylene rubber. 